The present invention relates, in general, to the testing of semiconductor devices and more particularly to the use of capacitance measurements to obtain the carrier concentration and profile depth for all pn semiconductor structures, and band discontinuities for heterojunction pn structures.
It is known to use capacitance measurements in nondestructive testing techniques to measure various characteristics or parameters of semiconductor devices. For example, a capacitance measuring arrangement which employs a mercury probe as a Schottky contact on a semiconductor is disclosed in U.S. Pat. No. 4,101,830. Such an arrangement can be used, for example, in providing a profile of the carrier concentration vs. depth of a pn semiconductor junction. This can be achieved by making capacitance measurements on the depletion layer of the semiconductor under reverse biased conditions. The distance measured into the semiconductor is controlled by the amount of reverse bias such that the greater the reverse bias voltage, the deeper the location of the measured carrier concentration. Well established formulae relate the measured capacitance and reverse biased voltage to the carrier concentration and depth.
A major shortcoming of this testing technique is that the profile depth is limited by the breakdown voltage of the semiconductor material. Once the breakdown voltage is exceeded, too much current begins to flow, thus making the capacitance measurements unreliable. Also, the established formulae and method of profiling do not correct for the additional capacitance of the pn junction.
U.S. Pat. No. 4,168,212 describes an invention which overcomes the problem of the reverse breakdown voltage. The invention set forth in this patent employs an electrolyte in place of the mercury probe which not only forms a Schottky contact with the semiconductor, but also provides a means of profiling to any depth by etching away semiconductor material. The arrangement achieves the measurement by alternately biasing the electrolyte solution to produce an anodic dissolution reaction between the semiconductor and the electrolyte that strips a small section of the semiconductor; and then stopping the etch to make capacitance measurements on the semiconductor material, typically with the depletion region unbiased. The cycle of etching and measuring is repeated until the desired depth is reached. Again, the measured capacitance is related to the carrier concentration and depth using the same well established formulae used in the arrangement set forth in U.S. Pat. No. 4,101,830. In addition, the depth calculation is augmented by a calculation related to the dissolution current.
For n-type semiconductors, the electrochemical system utilizes anodic dissolution at a fixed potential, at a rate determined by the availability of minority carriers which are created by illuminating the material. For p-type semiconductor material, no illumination is necessary. The integral of the dissolution current provides a depth that is used along with the depletion width obtained from the capacitance measurement to yield the depth scale of the profile plot. Although this electrochemical technique is not limited by the profile depth and does an excellent job of profiling isotype structures, the technique still does not correct for the capacitance of the pn junction, and performs a less accurate job of measuring the top layers of a pn semiconductor structure.
Techniques based on the two previously discussed devices have also been developed to measure the band discontinuity of heterostructures. In his paper "Measurement of Isotype Heterojunction Barriers by C-V Profiling" (Applied Physics Letter 36(4) 15 Feb. 1980), H Kromer describes a technique using isotype heterojunctions to measure band discontinuities. By applying conservation of charge and method of moments, Kromer developed a procedure to measure band discontinuity and interface charge. By adjusting parameters such as location of interface, size of interface charge, and, most importantly, the size of band discontinuity, Kromer's theoretical plot is matched against a measured profile obtained using either of the previously discussed inventions. The measured profile is obtained using either invention, however, the use of the nondestructive technique requires a thin top layer so that profiling of both sides of the heterojunction can occur before the breakdown voltage is exceeded.
Regardless of which apparatus is used, Kromer's technique suffers from several problems. The biggest problem is the reconstruction of the carrier concentration in the space charge region generated at the heterojunction. This reconstruction is difficult because of such anomalies as Debye smearing. As a result, the measured profile could suffer from errors not corrected by the technique. For example, any attempt to identify the location of the interface, an important parameter for Kromer's technique, would be prone to error. Another important parameter prone to error is the length of the space charge, which is necessary to establish the limits of integration in the technique. Even if an accurate measurement of the carrier concentration is obtained, numerous computations may be necessary to match the calculated profile from Kromer's technique with the profile measured with the C-V technique--each iteration requiring a new guess for the band discontinuity. The total computation to obtain a reasonable estimate to the band discontinuity could take up to 30 minutes on a standard desktop personal computer. Finally, a general limitation of the isotype technique is that large band discontinuities are needed to develop a space charge region large enough to influence the shape of the measured profile.